Semiconductor-metal hybrid nanoparticles (HNPs) manifest efficient light-induced charge separation [1-10]. Upon light excitation, the semiconductor component generates an electron-hole pair (exciton), followed by electron charge transfer to the metal domain, due to energy band alignment of the semiconductor-metal nanojunction. This, combined with the known catalytic characteristics of metal nanocrystals, made the HNPs leading candidates as photocatalysts for energy and environmental applications. In particular, HNPs were investigated as photocatalysts in water splitting for hydrogen generation under light irradiation, as means of directly converting solar energy to chemical energy stored in a fuel. Within these experiments, HNPs showed efficient hydrogen generation, whereas semiconductor nanocrystals have shown negligible activity.
Another photocatalytic application is the photo-reduction of carbon dioxide, using solar energy for promoting the reduction of green-house gas contents along with alternative sustainable avenues for production of methane and other hydrocarbon fuels.
Semiconductor nanocrystals have also been investigated for their use in diverse bioimaging and sensing applications, where they showed profound advantages over organic dyes and fluorescent proteins; mainly in terms of wavelength tunability, light sensitivity, and photochemical stability. Moreover, they have been suggested for photodynamic therapy and as light activated modulators of biological functions in neuronal stimulation applications, serving as alternatives to other approaches utilizing small molecule photoswitches, caged molecules enabling photorelease and opto-genetics tools.
Previous reports have shown that in the presence of oxygen, light excitation of semiconductor nanocrystals can result in the formation of reactive oxygen species (ROS) [11, 12]. The amount and type of ROS was found to depend on the composition and surface coating of the nanocrystals. Moreover, it was suggested that the produced ROS can be used to activate enzymes such as horseradish peroxidase (HRP) [13, 14]. This would depend on reduction of molecular oxygen with two protons and two electrons transferred from the conduction band of the semiconductor nanocrystals.